Our primary objective is to determine the molecular events that lead to the selective transcription of DNA and subsequent processing of viral and eukaryotic messenger RNA (mRNA). Poxviruses provide a unique and important system for such studies since the enzymes needed for transcription and mRNA modification are packaged within the core of infectious virus particles. During the past year, several important advances have been made. For the first time, the RNA polymerase has been solubilized and purified from vaccinia virus particles. DNA recombinant technology has been applied to vaccinia virus and genome segments have been cloned in phage and plasmids. This has allowed us to determine the presence of 30 tandem repetitions of a 70 bp sequence, present near each end of the genome, which are presumably used for DNA replication. Several early mRNAs and polypeptides have been mapped within the long 10,000 bp inverted terminal repetition. Significantly, no evidence of RNA splicing has been found. Continued studies on the enzymatic mechanism of capping mRNA has led to the purification of a cap specific mRNA (nucleoside-2-)-methyltransferase from HeLa cells and capping enzyme from wheat germ. In addition, a transfer RNA (cytosine-5-)-methyltransferase was purified from HeLa cells and shown to specifically methylate cytidine residues within the transition sequence between the extra arm and the righthand loop of tRNA.